Process for production of a metal substrate containing a protective coating

ABSTRACT

The present invention relates to a process for the production of metal substrates containing a protective coating, especially substrates based on iron, like steel, galvanized steel, or electrogalvanized steel, generally used in construction and in the automotive industry.

Aqueous Primer Compositions

Ingredients:

Aliphatic urethane-acrylate resins dispersed in an aqueous phase:

-   -   resin, marketed by UCB under the commercial name Ucecoat        DW7770®, and    -   resin, marketed by Synthopol Chemie under the commercial name        Syntholuc DRB 1577®.

Acrylic resins dispersed in the aqueous phase:

-   -   resin, marketed by UCB under the commercial name Duroxyn Vax®,        and    -   resin, marketed by Clariant under the commercial name Mowilith        LDM 7170®.

Photoinitiator:

-   -   alpha, alpha-dimethoxy-alpha-hydroxyacetophenone, marketed by        Ciba Geigy under the commercial name Darocur 1173®.

Inhibitor pigments:

-   -   calcium silicate, marketed by the GRACE Division under the        commercial name Shieldex C303®,    -   aluminum triphosphate, marketed by Safic Alcan under the        commercial name Kwhite KTC 720®kk,    -   zinc salt of an organic acid, marketed by Cognis under the        commercial name Alcophor 827®.

Fluorinated surfactant:

-   -   solution of polyether modified with polysiloxane, marketed by        Byk Chemie under the commercial name Byk 346®.

Thickener:

-   -   polyurethane thickener, marketed by Schwegmann under the        commercial name Schwego pur 8050®.

Anti-foaming agent:

-   -   mixture of polysiloxane and hydrophobic particles, marketed by        Byk Chemie under the commercial name BYK 028®.

Finishing composition:

-   -   hydroxylated polyester resin (in organic solvents)

Composition of Primer 1 Chemical nature Amount (parts by weight) UcecoatDW 7770 ® 80 Duroxyn Vax ® 20 Shieldex C303 ® 10 Kwhite KTC720 ® 5Alcophor 827 ® 0.5 Byk 346 ® 0.2 Schwego pur 8050 ® 0.3 Irgacure 500 ® 1

Composition of Primer 2: Chemical nature Amount (parts by weight)Ucecoat DW 7770 ® 80 Duroxyn Vax ® 20 Shieldex C303 ® 10 Kwhite KTC720 ®5 Alcophor 827 ® 0.5 Byk 346 ® 0.2 Schwego pur 8050 ® 0.3 Irgacure 500 ®12. Measurement MethodsSalt Spray Test

The anticorrosion performances of the samples are determined with thesalt spray test. After deposition of the protective coating, the coatedsteel sheets are subjected to the salt spray test, carried out accordingto Standard NF X 41-002. This test consists of spraying of a 5% byweight aqueous solution of NaCl, kept at a temperature of 35° C., in theform of a mist. The sheets are regularly examined at different stages(at 24 hours, 72 hours, 144 hours and 150 hours) of exposure to the saltspray. The presence of formed zinc salts is evaluated visually accordingto the following scale:

-   -   −: no zinc salts    -   +: traces of zinc salts    -   ++: formation of zinc salts (non-negligible)+    -   +++: abundant formation of zinc salts        Solvent Resistance Test

After application of the primer, and after application of a finishingcoat, the resistance of the coated sheets to a solvent like methyl ethylketone is evaluated.

For this purpose, the following procedure is used:

-   -   cotton is dipped in methyl ethyl ketone,    -   the impregnated cotton is then applied to the surface of the        sheet with a back-and-forth movement, and    -   the number of back-and-forth movements (or passes) carried out        on the appearance of softening and detachment of the protective        coating from the sheet is determined.

The higher the number of passes, the greater the degree of protection ofthe substrate, due to the coating.

Adherence Test

The degree of adherence of the protective coating to the surface of themetal substrate is determined as follows, according to Standard ISO2409:

-   -   incisions are made, forming a square grid pattern on the surface        of the protective coating (10×10 on a 1.5 cm² surface), then    -   adhesive tape with a width at least equal to the ruled surface,        carrying the reference 595TR1966, furnished by the 3M company,        is attached to the incised surface,    -   during peeling of the adhesive tape, the adhesive surface is        observed visually to determine the presence or absence of        coating flakes.        ERICHSEN Test

This test consists of deformation to 90% break of the coated substrate.This deformation is done on the back of the coated substrate.

EXAMPLE 1

Three galvanized steel sheets are degreased by immersion for 5 to 10seconds in a bath containing 3.75% by weight of Chemetall Parco 305E®,relative to the total weight of the bath. The pH of the bath is 13 to 14and the temperature 50° C. to 60° C.

The sheets are then rinsed with cold demineralized water, then dried ina drying cabinet at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activationtreatment by immersion in a bath of composition AFZ1 for one minute. Theplates are then dried in a drying cabinet for 5 minutes at 125° C.

In the five minutes following drying, the primer composition 1 is thenapplied, thus forming a moist primer film on the surface of the sheetsabout 10-12 μm thick.

The sheets coated with moist primer film are subjected to heat treatment(“flash off”) for 2 minute at 125° C., then crosslinking via radiationwith UV radiation. A crosslinked coating with a thickness of about 5-6μm is obtained.

The salt spray test is conducted on the sheets, along with the adherencetest, the solvent resistance test and the “ERICHSEN” test.

The coated sheets are then subjected to finishing treatment. Thisconsists of applying a finishing composition of the polyester/melaminetype, conventionally used in the paint industry. This finishingcomposition is applied by means of a laboratory rule, then crosslinkedby firing at 240° C. for 30 s.

After finishing treatment, the solvent resistance test, adherence testand “ERICHSEN” test are performed again.

The results obtained in these different tests are summarized in Table 1.

COMPARATIVE EXAMPLE C1

As a comparison relative to example 1, the same protective coating asdescribed in example 1 is applied to the galvanized steel sheets,proceeding in the same manner as described in example 1, except for theimmersion step in the bath of composition AFZ 1 based on fluorozirconicacid, which was omitted.

In the same manner as in example 1, the salt spray test is carried outon the sheets so coated (only before application of the finishing coat),along with the adherence test, the solvent resistance test and the“ERICHSEN” test.

The results obtained in these different tests are also summarized inTable 1. TABLE 1 “ERICHSEN” test Solvent Adherence of deformation Saltspray test resistance test test resistance Example 1 without Nooxidation 90 2 0 finishing with >100 0 0-1 finishing Comparative withoutOxidation 90 2 3 example 1 finishing with >100 5 1 finishing

Comparison of the results of the salt spray test of examples 1 and C1shows the presence of incipient oxidation when the specimen was notsubjected to initial activation treatment (example C1). This isillustrated by examples 1 to 3, in different stages of exposure to thesalt spray (at 24 hours, 72 hours and 144 hours, respectively). Thesefigures show that the sheets of comparative example 1 have whitelongitudinal traces (called “bleeding”) that correspond to the formationof zinc chloride and zinc hydroxide (white rust), whereas the sheets ofexample 1 do not show them. The resistance to salt spray is thereforebetter when the sheets have been subjected to activation treatment withfluorozirconic acid prior to deposition of the primer composition.

In addition, the adherence and behavior in the “ERICHSEN” deformationtest are significantly better with initial activation treatment. In thiscase (example 1), no delamination of the coating is observed. This isillustrated in FIG. 4. The sheets of FIG. 4 each show a lower part andan upper part: the lower part contains the primer film and the finishingcoat, whereas the upper part contains only the primer film. FIG. 4 showsthat at the deformation site, there is no longer any coating for thesheet of comparative example 1, both for the upper part (with finishingcoating), or for the lower part of the sheet (without finishingcoating). On the other hand, in the case of example 1, the coatingpersists (for the lower part and upper part with the finishing coat). Asa result, activation treatment with fluorozirconic acid improves theadherence of the protective coating to the metal substrate.

EXAMPLE 2

Galvanized steel sheets are degreased by immersion for 5 seconds in abath containing 3.25% by weight of Chemetall Parco 305E®, relative tothe total weight of the bath. The pH of the bath is 13 to 14 and thetemperature 50° C.

The sheets are then rinsed with cold demineralized water, then dried at125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activationtreatment by immersion in a bath of composition AFZ1 for 1 minute.

The plates are then dried in a drying cabinet for five minutes at 125°C.

In the five minutes following drying, primer composition 2 is thenapplied, thus forming a primer film on the surface of the sheet, havinga thickness of about 10-12 μm.

The sheets so coated with primer film are then subjected to heattreatment (“flash off”) for 2 minutes at 125° C., then crosslinking byirradiation with UV radiation (one pass at 10 m/min under two emittersof the RPC 80 W type under nitrogen).

The salt spray test, adherence test (“ruling”), the solvent resistancetest (A/R MEK) and the “ERICHSEN” of deformation resistance are thencarried out on the sheets so coated.

The coated sheets with the primer film are then finally subjected to thesame finishing treatment as in example 1.

A new solvent resistance test, adherence test and RXN “ERICHSEN” test ofdeformation resistance are then carried out.

The results obtained in the different tests are shown in Table 2.

COMPARATIVE EXAMPLE 2

As comparison relative to example 2, the same protective coating asdescribed in example 2 is applied to the galvanized steel sheets,proceeding in the same manner as described in example 2, except for theimmersion step in the bath of composition AFZ1 based on fluorozirconicacid, which was omitted.

The results of the different tests are also shown in Table 2, as forexample 2¹.¹Translator's note: sic—perhaps the French should have read “as forexample 1” because the reference is to the way results were shown in theprior table, which showed the results of example 1.TABLE 2 “ERICHSEN” test Solvent Adherence of deformation Salt spray testresistance test test resistance Example 2 without No corrosion 15 0 0finishing with X >100 0 1-2 finishing Comparative without Traces of 17 00 example C2 finishing white rust with X 90 1 2 finishing

Comparison of the results of the salt spray tests of examples 2 and C2is illustrated by FIGS. 5 and 6. These figures show that the sheets ofcomparative example 2 have white longitudinal traces (called “bleeding”)after 140 hours in a salt spray, whereas the sheets of example 2 do notshow them. The resistance to salt spray is therefore better when thesheets have been subjected to activation treatment with fluorozirconicacid prior to deposition of the primer composition.

Moreover, adherence and behavior in the deformation test (“ERICHSEN”test) of the sheets coated with a finishing coat are significantlybetter, when the substrate has been subjected to activation treatment.

EXAMPLE 3

Galvanized steel sheets are degreased by immersion for 5 seconds in abath containing 3.75% by weight of Chemetall Parco 305E®, relative tothe total weight of the bath. The pH of the bath is 13 to 14 and thetemperature 50° C.

In the same manner as in examples 1, 2, C1 and C2, the sheets are thenrinsed with cold demineralized water, then dried at 125° C. for 4minutes.

The degreased, rinsed and dried sheets are then subjected to activationtreatment by immersion in a bath of composition AFZ3 for 2 minutes.

The sheets are then dried in a drying cabinet (“flash off”) for 2minutes at 1 25° C.

In the five minutes following drying, the primer composition 1 is thenapplied, thus forming on the surface of the sheet a wet primer film,having a thickness of about 10-12 μm. The sheets coated with primer arethen subjected to the same finishing treatment as for example 1.

FIG. 7 shows the state of the coated sheets of example 3, after havingbeen subjected to the salt spray test for 150 hours.

COMPARATIVE EXAMPLE 3

As a comparison relative to example 3, the same protective coating asdescribed in example 3 is applied to the galvanized steel sheets,proceeding in the same manner as described in example 3, except for theimmersion step in the bath of composition AFZ3 based on fluorozirconicacid, nitric acid and cationic resin, which was omitted.

FIG. 8 shows the state of the coated sheets of example 3, after havingbeen subjected to the salt spray test for 150 hours.

The comparison of FIGS. 7 and 9 shows that the sheets of example 3 havesignificantly fewer white traces (“bleeding”) than those of comparativeexample 3, indicating that prior activation treatment imparts bettercorrosion protection to the primer film.

EXAMPLE 4

Electrogalvanized steel sheets are degreased by immersion for 5 secondsin a bath containing 3.75% by weight of Chemetall Parco 305E, relativeto the total weight of the bath. As for the preceding examples, the pHis 13 to 14, and the bath temperature 50° C.

The electrogalvanized steel sheets, in the same manner as desribed inthe preceding examples, are rinsed with cold demineralized water, thendried at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activationtreatment by immersion in the bath at ambient temperature of compositionAFZ2 for one minute.

The sheets are then dried in a drying cabinet (“flash off”) for 2minutes at 125° C.

In the five minutes following drying, the primer composition 1 is thenapplied, thus a film forming on the surface of the sheet with a wetthickness of about 10-12 μm. The sheets coated with primer are thensubjected to the same finishing treatment as for example 1.

COMPARATIVE EXAMPLE 4

As comparison, relative to example 4, the same protective coating as indescribed in example 4 is applied to the electrogalvanized steel sheets,proceeding in the same manner as described in example 4, except for theimmersion step in the bath of composition AFZ2 based on fluorozirconicacid, which was omitted. The comparison of the results of the salt spraytest (after exposure of 150 hours to salt spray) of examples 4 and C4 isillustrated in FIG. 9. This figure shows that the corrosion behavior issignificantly better when prior activation treatment occurs with acomposition based on the metal fluoroacid.

EXAMPLE 5

Electrogalvanized steel sheets are degreased by immersion for 5 secondsin a bath containing 3.75% by weight of Chemetall Parco 305E®, relativeto the total weight of the bath. As for the preceding examples, the pHis 13 to 14, and the temperature of the bath is 50° C.

The electrogalvanized steel sheets, in the same manner as the precedingexamples, are rinsed with cold demineralized water, then dried at 125°C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activationtreatment by application with a roll coater in a bath at ambienttemperature of composition AFZ2 for one minute.

The sheets are then dried in a drying cabinet (“flash off”) for 2minutes at 125° C.

In the 5 minutes following drying, the following primer composition isthen applied, thus forming a wet primer film on the surface of thesheet, having a thickness of about 6 μm: Rom UV: SM1530/140B DuroxyneVZX6127 18 Water 4.3 Anti-foaming agent byk 028 0.1 TEA 1 Schieldex C3033.6 K wite ktc 720 1.6 Ucecoat DW7770 72 Byk 346 0.3 Irgacure 500 1Schewgo pur 0.4

The sheets so coated then receive a film with 20 μm dry thickness of theanticorrosion primer W780-9735 from PPG by cathode electrodeposition.

A corrosion test VDA 621-41, an alkaline degreasing test and an Erichsentest are then carried out on the sheets.

COMPARATIVE EXAMPLE C5

For comparison, relative to example 5, the same protective coating asdescribed in example 5 is applied to the electrogalvanized steel sheets,proceeding in the same manner as described in example 5, except for theimmersion step in the bath of composition AFZ2 based on fluorozirconicacid, which was omitted.

Corrosion Behavior VDA 621-415 Example C5 Example 5 Red rust 30% 0-1%White rust 60% 15% Degreasing behavior Complete softening Good behaviorExample 5 Example C5 Erichsen adherence 3-4 0.1

Comparison of the results shows that activation (example 5) impartssignificantly better corrosion behavior, alkaline degreasing behaviorand adherence to the substrate.

This can be observed in FIGS. 9 and 10.

1. A process for the production of a metal substrate, containing aprotective coating, said substrate being based on iron, like steel,galvanized steel or electrogalvanized steel, or based on aluminum or itsalloys, said process comprising the following steps: (a) degreasing ofthe substrate with an alkaline solution, this step being optional whentreatment is carried out directly on a continuous galvanizing orelectrogalvanizing line; (b) rinsing with water, preferablydemineralized; (c) activation of the metal substrate with an activationcomposition based on at least one metal fluoroacid; (d) drying of themetal substrate; (e) application of an aqueous primer compositioncapable of being polymerized by irradiation with UV radiation or byelectron bombardment, to form film of aqueous primer composition atleast on 1 μm thick, preferably 3 to 5 μm thick, said primer compositioncontaining: a polymer binder, containing: 40 to 85 % by weight, relativeto the total weight of said primer composition, of at least oneunsaturated polymer resin, containing at least one acrylate group andcapable of being polymerized by irradiation with UV radiation or byelectron bombardment; 5 to 50 % by weight, relative to the total weightof said primer composition, of at least one saturated polymer resin notcontaining an acrylate group; and in the case, where polymerization isperformed by irradiation with UV radiation, 1 to 10 % by weight,relative to the weight of the said primer composition, of at least onephotoinitiator; a pigment paste, containing: a grinding resin; 1 to 20 %by weight, relative to the total weight of said primer composition, ofat least one corrosion inhibitor pigment; and optionally, at least onefiller; and water, preferably demineralized, as dispersion medium ofsaid primer composition, (f) heat treatment to eliminate the residualwater of the film of aqueous primer composition, and (g) crosslinking ofthe film of aqueous primer composition to obtain a primer film, having athickness of at least 1 μm, and preferably 3 to 5 pm, aftercrosslinking.
 2. The process according to claim 1, characterized by thefact that degreasing step (a) of the substrate is carried out with analkaline solution with a pH equal to or greater than 12, preferablyabout
 13. 3. The process according to claim 1 or 2, characterized by thefact that the alkaline solution used in degreasing step (a) of thesubstrate is a solution based on potassium hydroxide or sodiumhydroxide.
 4. The process according to any of the claims 1 to 3,characterized by the fact that the degreasing step (a) of the substrateis carried out by immersion in a bath of said alkaline solution, whosetemperature ranges from55 to 60° C.
 5. The process according to any ofthe preceding claims, characterized by.the fact that the metalfluoroacid of the activation composition of step (c) is chosen fromfluorozirconic acid, fluorotitanic acid and fluorosilicic acid.
 6. Theprocess according to claim 5, characterized by the fact that thefluoroacid is fluorozirconic acid.
 7. The process according to claim 6,characterized by the fact that the fluorozirconic acid is present in anamount of 0.1 g/L to 20 g/L, relative to the weight of the activationcomposition.
 8. The process according to claim 6 or 7, characterized bythe fact that the fluorozirconic acid is associated with a resin havingan epoxide structure, preferably a resin derived from polyepoxides,especially bisphenol A.
 9. The process according to any of the precedingclaims, characterized by the fact that the unsaturated polymer resin,containing at least one acrylate group of the primer composition of stepe), is a resin, chosen from epoxy-acrylates, urethane-acrylates,polyester-acrylates, polyester-urethane-acrylates, ether acrylates,amine acrylates or their mixtures.
 10. The process according to claim 9,characterized by the fact that the unsaturated polymer resin, containingat least one acrylate group, is a urethane-acrylate resin.
 11. Theprocess according to one of the preceding claims, characterized by thefact that the saturated polymer resin not containing an acrylate groupof the primer composition of step e) is chosen among dispersions oremulsions of acrylic, acrylic-styrene, acrylic-urethane and polyesterresin.
 12. The process according to any of the preceding claims,characterized by the fact that the photoinitiator of the primercomposition of step e) is chosen among benzyl ketones,dialkylacetylphenones, alpha-hydroxyalkylphenones, acylphosphine oxidesand benzophenone.
 13. The process according to any of the precedingclaims, characterized by the fact that the grinding resin of the primercomposition of step e) consists of a saturated polymer resin notcontaining an acrylate group of said primer composition.
 14. The processaccording to any of the preceding claims, characterized by the fact thatthe inhibitor pigment or pigments of the pigment paste of the primercomposition is (or are) chosen among calcium chromates, phosphates,borate, molybdates and silicates.
 15. The process according to one ofthe preceding claims, characterized by the fact that the primercomposition has a pigment concentration in CPV volume of inhibitorpigments ranging from 0.1 to 0.25, preferably 0.1 to 0.15.
 16. Theprocess according to one of the. preceding claims, characterized by thefact that the inhibitor pigment or pigments of the pigment paste of theprimer composition is (or are) associated with one or more corrosioninhibitor(s) in liquid form.
 17. The process according to claim 16,characterized by the fact that the corrosion inhibitor or inhibitors inliquid form is (or are) chosen among aqueous solutions of1-(benzo-thiazol-2-ylthio)succinic acid and aqueous solutions ofethylene methacrylate phosphate.
 18. The process according to any of thepreceding claims, characterized by the fact that the crosslinking ofstep g) is carried out under a UV lamp under nitrogen with an oxygenlevel lower than 200 ppm.
 19. The process according to any of thepreceding claims, characterized by the fact that it comprises, after thecrosslinking step g) of the aqueous primer composition film, a finishingstep h), comprising: application of finishing composition on the primerfilm, followed by crosslinking of said finishing composition to obtain afinishing coat.
 20. The process according to claim 19, characterized bythe fact that the finishing step h) is a finishing [process-translator],comprising: application of a finishing composition based on polyestermelamine or polyester urethane or polyvinyl difluoride, followed bycrosslinking of said finishing composition by heating in a dryingcabinet with a temperature in the range from 240-250° C., to obtain acrosslinked finishing coat.
 21. The process according to claim 19,characterized by the fact that finishing of step h) is cold finishing,comprising: application of a finishing composition based onurethane-acrylate, followed by crosslinking of said finishingcomposition by electron bombardment, to obtain a finishing coat.